Proximity sensors are used in a variety of consumer electronic devices, including in cell phones. In cell phones, the proximity sensor is used to detect when the user places the phone near their ear so that the touch screen can be disabled. Existing sensors for this application are optical proximity sensors, based on an infrared (IR) light emitting diode (LED) and one or more photodetectors. A typical proximity sensor (such as the Avago APDS-9950, Taos TMD2771, Capella Microsystems CM36683P) functions by detecting the intensity of IR light reflected from an object. The proximity sensor indicates when an object is near the sensor, typically within 100 mm+/−20 mm of the sensor, when the reflected IR level crosses a pre-determined threshold.
These proximity sensors suffer from a number of limitations. The first is that reflected IR intensity is a poor measure of proximity: the IR light reflected from two objects at the same distance will depend on the size and color of the object. This problem is very evident to cell phone users with dark hair and/or skin, who often find that their cell phone display and touch screen do not properly disable when they use the phone. A second problem is the high power required by the LED source, which can consume more than 10 milliwatts (mW). The stand-by power consumption of a typical cell phone is approximately 50 mW so the proximity sensor must be turned off most of the time to avoid draining the battery. A third problem is that optical proximity sensors can detect objects only over a limited range (approximately 100 mm). For applications in tablets, notebook computers, and monitors, it is desirable to be able to detect objects over a longer range (up to 600 mm or more). Finally, many applications require an accurate measure of distance (e.g. to the user's hand or head) that current optical proximity sensors simply cannot provide.
Accordingly, what is needed is a proximity sensor that provides an accurate measurement of the distance to an object, independent of the color or size of the object, that is low power (ideally below 1 mW), and that can operate at both short ranges (1 cm) and long range (>10 cm).